1. Field of the Invention
The present invention relates to an ignition timing control apparatus for an internal combustion engine, and more particularly to an improvement of an ignition timing control apparatus for prohibiting the ignition operation from being erroneously executed based on the noise added on an angle signal in an engine start-up condition.
2. Prior Art
As conventionally well known, an ignition timing control apparatus generates an ignition signal for actuating an ignition plug on the basis of an angle signal produced by a pickup sensor provided in a confronting relation to a signal rotor rotating in synchronism with a crank shaft of an internal combustion engine.
The signal rotor and the pickup sensor are generally disposed in the vicinity of the starter section of an engine. Thus, at the moment the engine is started up, the battery voltage is so largely fluctuated that electromagnetic noises are added onto the angle signal. Furthermore, the angle signal is further added vibration noises generated when the crank shaft starts rotating. The angle signal containing such noises tends to cause erroneous ignition signal, resulting in a mistaken ignition operation.
To solve this problem, the unexamined Japanese Patent Application No.HEI 4-325776/1992 discloses a technology of neglecting the angle signal for a predetermined time immediately after a voltage is applied to the starter (i.e. relay coil) of an internal combustion engine.
According to this prior art technology, the time required to neglect the angle signal is fairly long. Because, this time must be long enough to cover chattering which may be causes by the starter. This means that meaningless cranking will increase when no chattering is generated by the starter.
The Unexamined Japanese Utility Model Application No. SHO 63-26775/1988 discloses another ignition timing control apparatus which disables the power transistor of the ignition coil when the noise level detected by a noise detecting sensor provided near the electromagnetic pickup is larger than a predetermined value. Or, the power transistor is disabled when the battery voltage causes a large change more than a predetermined value.
According to such a prior art apparatus, it is feared that the ignition operation may be undesirably interrupted even if the angle signal is normal and sufficiently larger than the noise, because the ignition operation will be always prohibited when the noise level or the battery voltage variation exceeds the predetermined value.